This invention relates to leak testing of melt blown bottles, and particularly to improvements to the vacuum turntable for testing for leaks in the bottom surface of the bottles.
Referring now to FIGS. 1 and 2, many types of bottles 2, such as for sport drinks, bottled water, and many other substances, are formed in a melt blown process where a molded pre-form 4, formed originally of polyethylene terephthalate (PET) or similar materials is heated to within a predetermined range of the melting temperature of the material, and expanded by air pressure inside a mold to the final shape of the bottle. One particular point of the mold, the gate 6, is formed in the process of molding the pre-form 4 at the end of the pre-form opposite the neck 8. The gate 6 forms at the point where molten plastic is injected into the mold (not shown) during the manufacture of the pre-form 4. The area around the gate 6 is susceptible to cracks, splits or other defects that can result in open passages or leaks after the bottle 2 is formed. Because of the subtlety of the leak problems associated with the gate 6, it is particularly important to test this portion of the finished container 2 for leaks.
Conventionally, as shown at FIGS. 3-6, a turntable assembly 10 is provided with a flat turntable 12 which is rotated by a prime mover (not shown). The turntable 12 is provided with perforations 14 about its periphery. Bottles or containers to be tested for leaks are placed over these perforations 14. The vacuum for testing the bottles is provided from underneath the turntable 12 in the following manner. A stationary base plate 16 is connected to a source of vacuum by hoses 18 and fittings 20. To the base plate 16 is mounted a stationary vacuum plenum 22 by means of a number of jack screws 24. When assembled, the turntable 12 rotates, by means of an axle 26 or other suitable rotational means, resting just above the vacuum plenum 22. Sealing strips 28 are mounted to the vacuum plenum 22, to provide a certain level of seal in terms of passing the vacuum force through the openings 14 and to the bottom surface of the bottle 2.
This structure has several limitations and disadvantages, mainly relating to the fact that there must be a gap maintained between the sealing strip 28 and the bottom surface of the turntable 12, in order for the turntable to actually turn. At the same time, this gap between the sealing strips 28 and the turntable 12 must be small enough so as to ensure that sufficient vacuum is maintained through the openings 14 so as to conduct an effective test of the bottom of the bottle being tested. In the conventional apparatus, the ideal gap is between 0.008 inches and 0.010 inches. Any irregularity in the bottom surface of the turntable 12, or the overall flatness of the turntable 12, adversely affects this critical gap. If the gap becomes too large, vacuum is not maintained. If the gap becomes too small or closes entirely, the friction between the sealing strips 28 and the turntable 12 becomes so great that excessive power is necessary to be applied to rotate the turntable. This limitation is exacerbated as the diameter of the turntable is enlarged. That is, given the miniscule size of the ideal gap described above, as the diameter of the turntable is enlarged, it becomes more and more difficult to maintain this ideal gap, particularly as the diameter of the turntable is enlarged from two feet through four feet toward five feet. Larger diameters of turntables are desirable to support faster operation of bottle production lines while still maintaining the bottles in the turntable long enough to test for leakage.
Further, the fact that the gap is required to exist at all results in substantial leakage and loss of vacuum. Even the ideal gap described above allows a considerable amount of vacuum leakage thus reducing the effective vacuum at the base of the bottle 2. This leakage increases as diameter of the turntable 12 is increased, even when the ideal gap is maintained. When that leakage is coupled with the added leakage as the gap increases due to turntable runout, the vacuum source must be sized considerably larger to maintain an effective level of vacuum at the base of the bottle 2.
It will be noted that the vacuum plenum and sealing strips do not form a complete circle, but rather only the majority of a circle, there being a substantial gap at one portion of the circumference. This gap in the vacuum plenum and sealing strips exists because it is important that one portion of the turntable not have vacuum applied to the openings at that portion, so that the bottles may be easily removed from the turntable at some point in the sequence.
This invention relates to improvements to the apparatus described above, or to solutions to problems raised or not solved thereby.